1. Field of the Invention
This invention relates to a vehicle brake system having a sideslip prevention system.
2. Description of Related Art
Conventionally, brake systems having a sideslip prevention system are known in the art. As an example of one such brake system, see the brake system shown in Japanese Patent Application Laid-Open No. 7-117654. A few fundamental components of a brake system include a master cylinder which produces a brake fluid pressure corresponding to a force created when a driver steps on the brake pedal and a low-pressure reservoir having an essentially atmospheric pressure which provides brake fluid to a self-supplying pump. Also, most brake systems have a plurality of main conduits for transmitting a master cylinder pressure produced in the master cylinder to wheel cylinders. Pressure increase control valves are provided in each main conduit for controlling the increase of pressure applied to the wheel cylinders. Also provided in each main conduit are pressure decrease control valves that control the reduction of pressure in the wheel cylinders. The common brake system has a conduit which connects the self-supplying pump to a brake fluid delivery section between the pressure increase control valve and the pressure decrease control valve to supply brake fluid from the self-supplying pump, and a control valve for cutting off the master cylinder from the brake fluid delivery section when the pump draws and delivers brake fluid.
Brake systems have been modified to prevent or control sideslip. In sideslip prevention control, or trace characteristic increase control, when not braking, the self-supplying pump draws brake fluid directly from the low-pressure reservoir and delivers the brake fluid toward the wheel cylinders, thereby increasing the wheel cylinder pressure. When the driver begins to brake while in the process of sideslip prevention control, first a master cylinder pressure resulting from the pedal operation is applied to the wheel cylinders. After that, the master cylinder and the brake fluid delivery section are cut off by the control valve. As a result, the master cylinder pressure is sealed into the conduits on the wheel cylinder side. In this state, brake fluid is pumped to a wheel cylinder of a control object wheel.
However, in the above-described brake system, the brake fluid pressure formed in the conduits on the wheel cylinder side may become larger than the pressure formed by the brake fluid supplied from the port of the master cylinder in response to the brake pedal operation. For example, when sideslip control is carried out from the application of the brake pedal, a pressure resulting from a brake fluid amount supplied from the reservoir is added to the master cylinder pressure produced as a result of applying the brake pedal. Because of the additional pressure from the reservoir, the wheel cylinder pressure is higher than the master cylinder pressure.
In this case, after control termination counterflow of brake fluid accompanying brake fluid from the reservoir is carried out simultaneously, a large brake fluid pressure is applied to the port or seal parts of the master cylinder. Consequently, the performance characteristics of the master cylinder part or seal deteriorates.
Also, when rising brake fluid pressure inside the master cylinder due to brake fluid counterflowing occurs simultaneously with when the driver steps on the brake, the brake pedal may become difficult to depress.
Some vehicle brake systems have two piping systems in order to have both a sideslip prevention system and an anti-lock brake system as shown in Japanese Patent Application Laid-Open No. 6-87426. In this type of brake system, two pumps are provided to carry out sideslip prevention control and for supplying brake fluid from a reservoir to a first piping system and a second piping system. Braking forces are produced in the controlled wheels by driving these two pumps.
However, using separate pumps for a first piping system and a second piping system increases the complexity of the piping arrangement. Therefore, there is a demand for the ability to carry out sideslip prevention control with a single pump. In this case, applying braking forces to both the wheels in the first piping system and the wheels in the second piping system by means of a pump supplying brake fluid in a master reservoir to the first piping system is conceivable. That is, when in sideslip prevention control, the system increases the brake fluid pressure of the first piping system by delivering brake fluid supplied from the master reservoir to the first piping system, whereby a braking force is produced in the controlled wheel. If the controlled wheel is on the second piping system during sideslip prevention control, brake fluid supplied from the master reservoir is delivered to the first piping system. Delivering brake fluid to the first piping system increases the brake fluid pressure of a primary chamber of a master cylinder. The brake fluid pressure of a secondary chamber of the master cylinder is also increased due to the pressure increase within the primary chamber, whereby the brake fluid pressure of the second piping system is increased to produce a braking force in the controlled wheel.
However, because the primary chamber and the master reservoir are connected through an orifice, even when the primary chamber is increased in pressure there is a limit to that pressure increase. Consequently, even if the secondary chamber is increased in pressure along with pressure increase of the primary chamber, it is necessary to increase the brake fluid pressure in the second piping system by means of a pump for ABS control or the like thereafter. This deficiency increases the complexity of such a braking system.